1. Field of the Invention
The present invention relates to rotary encoders for detecting an angle of rotation, and more particularly to a rotary encoder including multiple calibration points allowing the rotary encoder to be calibrated by turning it through only a fraction of a complete 360-degree rotation.
2. Related Art
Rotary encoders are the sensor of choice for generating a digital output to accurately measure rotational motion. Encoders are often attached to high performance spindles in systems such as high precision machine tools and laser scanners. They are used to measure rotational parameters such as shaft angle position, velocity and direction of rotation.
A rotary encoder typically has two or three signal channels. One of these channels is generally a one-pulse-per-revolution index channel, and the other channels are generally multiple-pulse-per-revolution data channels. The number of pulses per revolution on a data channel is typically between a few hundred and a few thousand depending upon the resolution required for fine position control. These signals are often fed into a control system to provide real-time information regarding the position, velocity and rotational direction of the spindle.
The index channel pulse is used to indicate the beginning of each spindle revolution, and the data channel(s) signal is used to indicate the speed of the rotor and the angular position of the rotor within a revolution. For some types of encoders (such as a quadrature type encoder) there is an additional channel which provides information on the direction of the spindle.
FIGS. 1A and 1B illustrate portions of a prior art rotary encoder. The rotary encoder illustrated in FIGS. 1A and 1B includes rotatable disk 100, which is coupled to shaft 101. Shaft 101 is coupled to a rotational input, such as a spindle for a machine tool, so that rotating the spindle cases shaft 101 and disk 100 to rotate. Disk 100 includes two channels comprising circumferentially extending rows of transparent openings through which light can pass. In the illustrated example, the inner channel is an index channel 102, with a single opening (pulse) per revolution specifying an index position. The outer channel is a data channel 104 with a large number of openings (pulses) specifying incremental angular displacements of disk 100. Rotatable disk 100 is typically composed of glass, and the openings which form index channel 102 and data channel 104 are typically etched in rotatable disk 100.
In the illustrated example, information from index channel 102 and data channel 104 is retrieved optically. Light emitting device 106 generates light, which passes through index channel 102 and feeds into light receiving device 112. The signal from light receiving device 112 passes into signal processing device 114, which detects and decodes the index pulse from index channel 102. Similarly, light emitting device 108 generates light, which passes through data channel 104 and feeds into light receiving device 110. The signal from light receiving device 110 passes into signal processing device 114, which detects and decodes an angular displacement signal from data channel 104. Light emitting devices 106 and 108 are typically implemented using light emitting diodes (LEDs) or lasers. Light receiving devices 110 and 112 are typically implemented using photodiodes. Signal processing device 114 is typically implemented with a device controller or a microprocessor.
One problem with the above-described rotary encoder design is that it requires up to a complete revolution of the rotary encoder to detect the pulse on the index channel in order to calibrate the rotary encoder. Turning the rotary encoder through a complete revolution may not be possible or may be inconvenient for certain applications, such as a wind direction indicator, a rudder position indicator or a joystick.
What is needed is a rotary encoder that can be calibrated without having to turn it through a complete revolution.
A rotary encoder with multiple index points has previously been disclosed. See U.S. Pat. No. 5,130,536, entitled "Optical Rotary Encoder with Indexing," to Sato et al. However, the rotary encoder disclosed in Sato was developed to facilitate the firing of spark plugs, not for purposes of calibration. Hence, the multiple index points in Sato do not contain information specifying angular positions for the index points for purposes of calibration. In FIG. 2 of the Sato patent, the zero angle index point contains a marker with two openings to differentiate it from the other index points. The other index points merely include a marker with a single opening, and these other index points cannot be differentiated from one another for purposes of calibration. (See FIG. 2) Hence, it is necessary to turn the rotary encoder disclosed in Sato through almost a complete revolution in order to calibrate it. (Strictly speaking, it is possible to calibrate this encoder by turning it though 1-1/N revolutions, where N is the number of index points.)